NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Theoretical Investigation on the Properties of Energy Materials

Qianfan Zhang, Stanford University

Associated NERSC Project: Searching and designing new materials for topological insulators (m1131), Principal Investigator: Shoucheng Zhang

Search for novel energy material has become an important goal in material sciences. Recently, silicon nanowires (SiNWs), as well as other Si nanostructures, including crystalline-amorphous core−shell SiNWs, carbon-amorphous Si core−shell NWs, Si nanotubes, porous Si particles and an ordered macroporous carbon-Si composite have been demonstrated as ultrahigh capacity lithium ion battery negative electrodes; this opens up exciting opportunities for energy storage devices. However, experimental investigations of these Si nanostructures have mainly been focused on electrochemical cycling of Si−Li compounds and phase transitions during Li insertion. Although the Si−Li compound can be accurately analyzed, the fundamental interaction between Li and Si atoms and the microscopic dynamic process during Li insertion still remain unknown. Based on our former work, we will continue the theretical study of Li insertion in different kinds of Si nanostructures, including SiNWs, Si nano-shell… Our study will be very important in revealing the nature of quantum confinement effects and the fundamental mechanics of Li insertion and diffusion, and at the same time, give suggestion on promising routes for future electrochemical applications.

We propose a program to theoretically investigate the properties of energy materials, like Si nanowires, Si nano-particles and Si nano-shell, by DFT (density functional theory) computational method, by designing new theoretical model, and by working closely with the experimental colleagues to measure its novel properties.

Study Li insertion and diffusion in different kinds of Si nano-structures

We already have a systematic study of the single Li insertion in SiNWs with different diameters along different axis orientations within the DFT framework. It is the first step for further computational simulation on Li insertion in SiNWs, as well as other Si nanostructures. We will continue our study on this project, and investigate the fundamental mechanism of Li-Si interaction in SiNWs.

Experimental investigations have not paid much attention to Li insertion behavior in SiNWs with different orientations and different sizes, especially when the Li doping ratio is very low and the SiNW is ultrathin. Although some theoretical works have focused on the Li-SiNW interaction, the effect of anisotropy on the interaction of Li with SiNWs of different orientations still remains unclear. Our previous work also neglects the detailed discussion on this aspect. We will study systematically the relationship between Li insertion behavior and SiNW orientation. We will perform first-principle simulations on [110], [100], [111] and [112] SiNWs with single Li impurity, and calculate the electronic structure in order to study the impact of quantum confinement and the reason for different Li insertion behavior in different orientated SiNWs.

Other Si nano-structures, like Si nanotube or Si nano-shell, are also good candidates for Li battery anode. We want to study Li insertion and diffusion behavior in various Si nano-sructures using the method that is similar with SiNW study, and calculate Li binding energy and diffusion barrier in these materials to see the distinct Li insertion behavior between different nano-structures. The other important aspect is the size effect of nano-structure, because as the size decrease, the surface Si atoms weight much and quantum confinement makes great effort. We expect the Li insertion and diffusion behavior can be quite different in the structures with small size.

Strain effect and doping effect in various Si nano-structures

One of the most important anisotropic effects in confined system relates to the response to external strain. Imposing strain is a common way to change the properties of material and improve device performance. SiNW strain studies, both experimental and theoretical, have shown remarkable changes in SiNW properties, like great enhancement of carrier mobility and significant modification of band structure, especially when the diameter is small. Recently, a theoretical study indicated that a strained SiNW can open up a new avenue for application in solar cells. Therefore, it can be predicted that the strain can also affect the Li insertion in SiNWs for energy storage application. Furthermore, strain exists naturally in real SiNWs and its occurrence is almost unavoidable during the grow process. Therefore, the theoretical simulation of Li insertion in strained SiNWs is also helpful to assist in experimental understanding. Our ab initio simulation will also focus on Li insertion and diffusion in SiNW under strain effect. For SiNWs, the strain can be applied to either axis direction or radial direction, and it is predicted that the influence can be quite different; On the other hand, different orientated SiNWs have different response to strain. We will study these effects in details.

Strain effect in other Si nano-structures is also interesting. It can be predicted that the impact of strain is dependent on the size and configuration of these low-dimension system. We will do ab initio simulation on nano-shell, nanotube or other nano-particles, to see which structure is ideal for strain effect application.

N or P doping is another important way to change the properties of semiconductor materials. Some previous work theoretically studied N or P doping in SiNWs and investigated its effect on electronic band structure. We suggest to study Li insertion in N or P doped SiNWs to see whether such doping can affect Li behavior in SiNWs. Our plan is to substitute one Si atom in SiNWs by doping atom B, N or P and see the Li insertion and diffusion behavior near the doping atom.